The finding, from a large, randomized, placebo-controlled trial, is a step toward U.S. approval for the cell-based manufacturing method, which has the potential to produce a more reliable and flexible flu vaccine supply.

The report comes only a few months after a study that showed another cell-based vaccine and one grown in chicken eggs -- the standard manufacturing method -- had similar efficacy rates compared with placebo.

The current study also showed that the so-called correlates of protection for the new vaccine -- markers that are used to test a vaccine's ability to prevent disease -- are similar to those used to evaluate standard egg-based vaccines, according to lead author P. Noel Barrett, PhD, of Baxter BioScience in Orth/Donau, Austria, and colleagues.

The vaccine was well-tolerated, the researchers reported online in The Lancet, with most adverse events mild and transient and no serious side effects reported.

One advantage of the cell-based method is faster production time, according to W. Paul Glezen, MD, of Baylor College of Medicine in Houston. In an accompanying commentary, Glezen noted that culture-based manufacturing could shave 10 weeks off the time needed to make a vaccine from eggs.

The time saving "might be crucial in a pandemic alert," he said.

He also noted that the process of growing the vaccine in eggs introduces changes to the hemagglutinin antigen -- the most important surface protein on a flu virus and the target of the induced antibody response.

There is evidence that growing the vaccine in mammalian cells -- the Vero cells used for the new vaccine are derived from African green monkeys -- might yield a medication that is better adapted for use in humans, Glezen argued.

During the 2008/2009 vaccine season, Barrett and colleagues randomly assigned 7,250 adults ages 18 through 49 in 36 U.S. centers to get either the vaccine or a placebo. The primary outcome of the trial was prevention against cell culture-confirmed infection by one of the three vaccine strains.

All told, the researchers tested 104 specimens from 103 participants; 73 of them were matched to one of the three vaccine strains, including 63 matched to the predominant influenza A/H1N1 strain, six to influenza A/H3N2, and four to influenza B.

Of those, 60 cases occurred among participants who received a placebo and 13 among those who got the vaccine, the researchers reported, yielding an efficacy rate of 78.5%.

Immunogenicity, as measured by hemagglutination inhibition titers 21 days after vaccination, meet U.S. standards for all three strains, the researchers reported.

Analysis suggested that titers of 1:15 correlated with the immunity induced by the vaccine, they reported. Titers of at least 1:10 are usually regarded as protective.

Barrett and colleagues cautioned that the vaccine was not compared directly with an egg-derived vaccine.

Additionally, they said, the efficacy reached significance only for the H1N1 strain because of the low number of H3N2 isolates and a "substantial mismatch" between the B strain in the vaccine and isolated B strains.

The study had support from the U.S. Office for Preparedness and Response, Biomedical Advanced Research and Development Authority, under a contract to DynPort Vaccine Company.

Barrett and several other authors are employees of Baxter BioScience, which makes the vaccine. Some authors are also employed by DynPort.

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